1. Field
The present disclosure describes a semiconductor device having a self-aligned base contact and a narrow emitter.
2. Description of Related Art
In heterojunction bipolar transistor (HBT) technology, two parameters that are ideally optimized are the transit frequency (ft) and the maximum frequency (fmax) of the HBT. The transit frequency is a measure of the frequency at which the small signal current gain of the HBT is approximately unity. The maximum frequency of the transistor is the frequency at which the input power equals the output power.
One technique for optimizing the maximum frequency is to reduce the emitter width and reduce the spacing between the emitter and the base contact. Previously, to reduce the spacing between the emitter and the base contact, a self-aligned base contact was used in the HBT. When depositing a self-aligned base contact, a portion of the emitter under the emitter contact is removed so as to undercut the emitter contact. Undercutting the emitter contact forms an overhang that is used as a mask to deposit the base contact. Shown in FIG. 1 is an HBT with a self-aligned base contact. The HBT comprises a collector 10, base 20, emitter 30, and emitter contact 40. The emitter contact 40 is undercut so as to remove a portion of the emitter 30. Then using the emitter contact 40 as a mask, the base contact 50 is deposited. By monitoring the undercut, the base contact 50 can be closely deposited to the emitter 30. Also, removing a portion of the emitter 30 helps to reduce the width of the emitter 30. However, undercutting the emitter contact 40 makes use of a crystallographic wet chemical etchant, which has some drawbacks.
One drawback is that the emitter height to width ratio must be carefully monitored during the undercutting process. While submerged in the wet-etchant, if the emitter width becomes too narrow, the emitter contact may “lift off” from the emitter and float away. As a result, there is a lower limit on the width of the emitter that may remain submerged in the wet-etchant, which places a practical limit on the maximum frequency on the device. In current state of the art technology this lower limit is around 0.5 μm. Another drawback is that the spacing between the base contact and emitter sidewall can be difficult to control. If the base contact is placed too close to the emitter, undesirable leakage currents may form resulting in low beta values, or the base contact may short with the emitter. Yet another drawback is that the undercutting process results in an undercut which can be more severe at the emitter/emitter contact interface than at the emitter/base interface. This results in the emitter resistance being increased.
As a result, there is a need for a semiconductor device in which the width of the emitter and the spacing between the emitter and base contact is not determined by the aforedescribed undercutting process.